Improved and Reduced Performance of Cu- and Ni-Substituted Co3O4 Catalysts with Varying CoOh/CoTd and Co3+/Co2+ Ratios for the Complete Catalytic Oxidation of VOCs

Sustainable recovery Co3O4-based catalysts from spent lithium-ion batteries for preferential CO oxidation

Retired Lithium-ion batteries (LIBs) present significant challenges related to environmental pollution, making the recycling of battery materials essential for environmental sustainability and energy recovery. Cobalt has been identified as an effective catalyst for oxidation reactions; accordingly, Co3O4 was recovered from batteries directly for use in preferential CO oxidation (CO-PROX). To enhance the low-temperature catalytic activity, supported CuO/Co3O4 catalysts were synthesized, resulting in a remarkable boost in catalytic performance, with thorough CO conversion (T100%) at 160 °C. However, the way a reaction occurs remains unclear, and the active sites for CO adsorption require further investigation. Characterization techniques and density functional theory (DFT) calculations indicated that CuO/Co3O4 exhibits higher lattice oxygen (Olatt) content and strong metal-support interactions, which facilitate CO adsorption while suppressing hydrogen activation. Moreover, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) analysis was used to analyze reaction intermediates, verifying that CuO/Co3O4 significantly enhances both CO conversion and CO2 selectivity. This research presents a viable strategy for the recycling of retired LIBs and the advancement of efficient catalysts for CO-PROX.

Synergistic catalytic degradation of benzene and toluene on spinel MMn2O4 (MCo, Ni, Cu) catalysts

Owing to the complexity of multicomponent gases, developing multifunctional catalysts for synergistic removal of benzene and toluene remains challenging. The spinel MMn2O4 (MCo, Ni, or Cu) catalysts were successfully synthesized via the sol-gel method and tested for their catalytic performance for simultaneous degradation of benzene and toluene. The CuMn2O4 sample exhibited the best catalytic performance, the conversion of benzene reached 100 % at 350 °C, and toluene conversion reached 100 % at 250 °C. XRD, N2 adsorption-desorption, HRTEM-EDS, ED-XRF, Raman spectroscopy, H2-TPR, NH3-TPD, O2-TPD and XPS were used to characterize the physical and chemical properties of MMn2O4 catalysts. The excellent redox properties, high concentration of surface Mn4+, and adsorption of oxygen species over the CuMn2O4 sample facilitated the simultaneous and efficient removal of benzene and toluene. Additionally, in situ DRIFTS illustrated the intermediate species and reaction mechanism for the synergetic catalytic oxidation of benzene and toluene. Notably, as an effective catalytic material, spinel oxide exhibited excellent synergistic degradation performance for benzene and toluene, providing some insight for the development of efficient multicomponent VOC catalysts.

Promoting Low-Temperature Toluene Oxidation via Pt-O-Fe Interfacial Sites in a Pt/CuO-Fe3O4 Catalyst

Electronic metal-support interaction (EMSI) has been widely explored in the catalytic degradation of volatile organic compounds (VOCs) owing to the formation of special interfacial sites. Herein, the EMSI effect was engineered by constructing the serial Pt catalysts supported on CuO-Fe3O4 bimetal oxide (Pt/CFO). Among them, the 0.5Pt/CFO catalyst with 0.5 wt % Pt loading exhibited an outstanding catalytic activity, with T90 (the temperature of 90% toluene conversion) lowered to 185 °C, and displayed excellent stability and water resistance. Comprehensive physicochemical characterizations revealed that an evident electron transfer occurred via the interface structure (Pt-O-Fe), producing the positively charged Pt (Ptδ+) and abundant Fe2+ species. Notably, the increased electron density around the Fe species weakened the Fe-O bond and thus activated the surface lattice oxygen (Olatt). Further, temperature-programmed desorption experiments and in situ diffuse reflectance infrared Fourier transform spectroscopy results demonstrated that the electron-deficient Ptδ+ was conducive to the adsorption and activation of toluene at low temperature. Consequently, the deep oxidation of toluene was achieved with the participation of Olatt, benefiting from the Ptδ+-O-Fe2+ interfacial sites with synergistic catalysis for toluene adsorption and oxygen activation. This work provides an interesting idea to explore the relationship between the electron transfer effect and highly efficient VOC abatement.

Engineering oxygen vacancies in acid-etched MgMn2O4 for efficiently catalytic benzene combustion: Synergistic activation of gaseous oxygen and surface lattice oxygen

The synergistic activation of gaseous oxygen and surface lattice oxygen is essential for designing highly efficient catalysts to eliminate VOCs. Herein, an effective acid treatment was carried out to create more oxygen vacancies by modulating the electronic structure of MgMn2O4 spinels and MgMnOx mixed oxides. The acid-treated MgMn2O4 exhibited outstanding catalytic performance, with the reaction rate of benzene rising by 8.55 times at 200 °C. After acid treatment, MgMn2O4 partially retained its spinel structure, while Mn2O3 in situ grew on the surface due to the selective removal of Mg2+. The transformation of Mn-O-Mg into Mn-O weakened the strength of adjacent Mn-O bonds, thereby promoting the release of surface lattice oxygen and the regeneration of oxygen vacancies. In addition, acid-treated MgMn2O4 facilitated the adsorption and activation of gaseous oxygen. In situ DRIFTS analysis proved that the synergistic activation of gaseous oxygen and surface lattice oxygen accelerated the conversion of intermediates, thus contributing to the efficient degradation of benzene.

Enhancing Catalytic Removal of Autoexhaust Soot Particles via the Modulation of Interfacial Oxygen Vacancies in Cu/CeO2 Catalysts

The purification efficiency of autoexhaust carbon strongly depends on the heterogeneous interface structure between active metal and oxide, which can modulate the local electronic structure of defect sites to promote the activation of reactant molecules. Herein, the high-dispersion CuO clusters supported on the well-defined CeO2 nanorods were prepared using the complex deposition slow method. The formation of heteroatomic Cu+-Ov-Ce3+ interfacial structural units as active sites can capture electrons to achieve activation of the NO and O2 molecules. Among all of the synthesized catalysts, the Cu10/CeO2 catalyst exhibits superior catalytic performance (T50 = 351 °C) along with remarkable tolerance to H2O and SO2 in the removal of soot particles. Through a combination of comprehensive characterizations and density functional theory calculations, it is proposed that the interfacial Cu+-Ov-Ce3+ site, acting as an electron enrichment center, can capture electrons from the Cu d-band and Ce d/f-band to obtain high delocalized electron density, and then enhance the oxidation of NO to NO2, which plays a crucial role in the NOx-assisted catalytic mechanism for soot oxidation. This study presents a novel strategy for developing highly efficient catalysts that exhibit resistance to H2O and SO2, aimed at enhancing the removal of soot particles.

Insight into the Synergistic Effect of Binary Nonmetallic Codoped Co3O4 Catalysts for Efficient Ethyl Acetate Degradation under Humid Conditions

The synthesis of high-performance catalysts for volatile organic compounds (VOCs) degradation under humid conditions is essential for their practical industrial application. Herein, a codoping strategy was adopted to synthesize the N-Co3O4-C catalyst with N, C codoping for low-temperature ethyl acetate (EA) degradation under humid conditions. Results showed that N-Co3O4-C exhibited great catalytic activity (T 90 = 177 °C) and water resistance (5.0 vol% H2O, T 90 = 178 °C) for EA degradation. Characterization results suggested that the C, N codoping weakened the Co-O bond strength, increased surface Co3+ and Oads species, and improved the low-temperature redox ability and the mobility of lattice oxygen species, which boosted the catalytic performance of N-Co3O4-C for EA degradation. Meanwhile, the N-doping-induced oxygen vacancies could interact with water vapor to generate extra active oxygen species, which enhanced the water resistance. Importantly, based on a series of characterization technologies, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and theoretical calculations, the synergistic effect of C, N codoping was systematically investigated and elucidated. The C doping induced the increase of surface area and the weakening of Co-O bond strength, which improved EA adsorption and lattice oxygen species activation to dissociate and oxidize EA, forming the key intermediate, acetate acid. N doping enhanced the adsorption and activation of gaseous oxygen species to form active oxygen species, attacking and breaking the C-C bond in acetate acid to accelerate EA deep oxidation, which synergistically facilitated EA degradation.

α-MnO2 catalysts with efficient ozone-catalyzed decomposition under high humidity conditions

Ground-level ozone pollution poses significant risks to ecosystems and human health and requires effective control measures. This study focused on the monolithic ozone degradation catalyst based on powdered α-MnO2 and comprehensively investigated its catalytic performance, moisture resistance, and stability. The monolithic catalyst achieved the optimal catalytic activity with an ozone conversion rate of 99% after being calcined at 400 °C for 3 hours. The detailed characterization of the catalyst properties at pH = 1, 4, and 7 revealed the adverse effects of residual acid ions on the catalyst activity. The catalyst at pH = 7 had more oxygen vacancies, which was related to the reduction of sulfate ion residues and the exposure of more active sites during the washing process. At pH = 7 and a space velocity of 900 000 h-1, the conversion rates of α-MnO2 to 18 ppm ozone reached 100% and 95% within 3 hours under 90% relative humidity and dry conditions, respectively. In addition, the monolithic catalyst exhibited significant moisture resistance and performed well in continuous alternating humidity cycle tests and sustained high humidity. It still maintained 90% ozone decomposition efficiency after 3 hours of testing under high humidity conditions. Meanwhile, the α-MnO2 monolithic catalyst showed excellent stability, with an ozone conversion rate exceeding 99% during the 50 - hour test period. These findings highlight the great potential of the α-MnO2 monolithic catalyst in ozone removal applications.

Enhanced sensing of dinotefuran in foods based on BC/ZnCo MOF@PBA nano-enzyme induced MIECL sensor

Natural enzymes can increase the signal of electrochemiluminescence. However, they are expensive and environmentally demanding. Here, the hollow prussian blue analogues decorated and biomass-derived carbon doped ZnCo metal-organic framework nano-enzyme was designed via self-assembly method. The BC/ZnCo MOF@PBA with peroxidase activity boosts the ECL intensity by catalyzing H2O2 to produce more free radicals as intermediates in the ECL reaction. Then, the molecularly imprinted polymer was synthesized by in-situ electropolymerization using o-aminophenol as the functional monomer and dinotefuran as the template molecule to form the MIECL sensor. This MIECL sensor exhibited high sensitivity for dinotefuran detection with a good linear relationship range from 0.01 μM to 100 μM, and a low detection limit of 0.0046 μM, which was better than the reported dinotefuran sensor. Additionally, this MIECL sensor has good selectivity, stability, repeatability, etc. Lastly, the MIECL sensor performed well in the fruit sample detection.

Trimetallic FeNiCu Atomic Clusters Supported on Carbon Matrix: Highly Active Catalysts for C3H6-SCR of NO

C3H6-SCR denitrification technology faces catalyst deactivation problems and low catalytic performance at medium-low temperatures. This study utilized the intermetallic synergies to prepare atomic cluster catalysts (FeNiCu/NC) by anchoring Fe-Ni-Cu on a carbon matrix to enhance the C3H6-SCR performance at medium-low temperatures. The synergistic effect of the Fe-Ni-Cu is reflected in the differences in the physicochemical properties of the catalysts, which is proved by several characterization techniques. Results showed that the FeNiCu/NC catalyst had a larger surface area (541.4 m2/g) and there were no metal oxides on the surface of the catalyst but abundant defective sites that anchored Fe/Ni/Cu atoms through N atoms to form M-Nx active sites and atomic clusters. The hollow carbon morphology provides sufficient active sites for C3H6-SCR. The coordination environments of active sites were M-Nx-C, Fe2/FeCu2/FeNi2, Ni3/NiFe/NiCu2, and Cu4/CuFe2/CuNi2, where the synergistic action of trimetal leads to the presence of Fe-Ni-Cu-Nx-C. The synergistic action of the Fe-Ni-Cu significantly improved the C3H6-SCR performance at medium-low temperatures. The FeNiCu/NC exhibited an 81% NO conversion at 150 °C under 2% O2, 15% and 20% higher than FeNi/NC and FeCu/NC catalysts, respectively. Even at 4% O2, the FeNiCu/NC catalyst was active to remove 78% NO and achieve a 93% N2 selectivity at 150 °C and maintained a 100% NO conversion at 300-425 °C. The DRIFTS results demonstrated that NO and C3H6 could combine with active O at metal cluster, M-Nx, or defective oxygen sites to produce various intermediate species, wherein acetates and nitrates were the main active intermediates. Based on the DRIFTS results, a reaction pathway for C3H6-SCR over the FeNiCu/NC catalyst was proposed.

Spinel-Based Catalysts That Enable Catalytic Oxidation of Volatile Organic Compounds

Volatile organic compounds (VOCs) have caused serious harm to human health and ecological environment, and have received much attention in recent years. Despite the successful applications of catalytic combustion of VOCs as the core technology of VOCs removal in industry, the development of efficient catalysts that can mineralize VOCs into nontoxic CO2 and H2O at low temperatures remains a great challenge. Recent studies show that spinel-based materials as efficient catalysts were extensively used in the catalytic oxidation VOCs field due to their synergistic effect, manifold compositions, and electron configurations. However, most of the pollutants are complex, consisting of multiple VOCs, water vapor, CO2, SO2 and other substances, which presents a significant challenge in constructing highly active and stable catalysts. To meet the future demand for efficient catalysts capable of removing various types of VOCs, it is urgent to rationally design and scientifically prepare spinel catalysts based on existing knowledge. This work reviews the research and development of various spinel catalysts with an emphasis on their catalytic performance in VOCs oxidation. The catalytic performance of spinel-based catalysts for different sorts of VOCs was summarized and compared. Moreover, the effects of the reaction conditions on the catalytic performance of spinel-based catalysts were examined to accommodate complicated operating conditions. Subsequently, the regulation of spinel oxides in structure and defect was coherently reviewed to guide the development and design of efficient catalysts. Especially, the research techniques for the reaction mechanism over spinel catalysts were displayed to better deepen the understanding of catalytic oxidation of VOCs. Finally, the current development and challenges were proposed and put forward for future research. This review provided a systematic understanding of the VOCs oxidation over spinel-based catalysts and offered guidance for the development of high-performance catalysts for VOCs elimination.

A Critical Review of Deep Oxidation of Gaseous Volatile Organic Compounds via Aqueous Advanced Oxidation Processes

Volatile organic compounds (VOCs) are considered to be the most recalcitrant gaseous pollutants due to their high toxicity, diversity, complexity, and stability. Gas-solid catalytic oxidation methods have been intensively studied for VOC treatment while being greatly hampered by energy consumption, catalyst deactivation, and byproduct formation. Recently, aqueous advanced oxidation processes (AOPs) have attracted increasing interest for the deep oxidation of VOCs at room temperature, owing to the generation of abundant reactive oxygen species (ROS). However, current reviews mainly focus on VOC degradation performance and have not clarified the specific reaction process, degradation products, and paths of VOCs in different AOPs. This study systematically reviews recent advances in the application of aqueous AOPs for gaseous VOC removal. First, the VOC gas-liquid mass transfer and chemical oxidation processes are presented. Second, the latest research progress of VOC removal by various ROS is reviewed to study their degradation performances, pathways, and mechanisms. Finally, the current challenges and future strategies are discussed from the perspectives of synergistic oxidation of VOC mixtures, accurate oxidation, and resource utilization of target VOCs via aqueous AOPs. This perspective provides the latest information and research inspiration for the future industrial application of aqueous AOPs for VOC waste gas treatment.

Improving benzene catalytic oxidation on Ag/Co3O4 by regulating the chemical states of Co and Ag

Silver (9 wt.%) was loaded on Co3O4-nanofiber using reduction and impregnation methods, respectively. Due to the stronger electronegativity of silver, the ratios of surface Co3+/Co2+ on Ag/Co3O4 were higher than on Co3O4, which further led to more adsorbed oxygen species as a result of the charge compensation. Moreover, the introducing of silver also obviously improved the reducibility of Co3O4. Hence the Ag/Co3O4 showed better catalytic performance than Co3O4 in benzene oxidation. Compared with the Ag/Co3O4 synthesized via impregnation method, the one prepared using reduction method (named as AgCo-R) exhibited higher contents of surface Co3+ and adsorbed oxygen species, stronger reducibility, as well as more active surface lattice oxygen species. Consequently, AgCo-R showed lowest T90 value of 183°C, admirable catalytic stability, largest normalized reaction rate of 1.36 × 10-4 mol/(h·m2) (150°C), and lowest apparent activation energy (Ea) of 63.2 kJ/mol. The analyzing of in-situ DRIFTS indicated benzene molecules were successively oxidized to phenol, o-benzoquinone, small molecular intermediates, and finally to CO2 and water on the surface of AgCo-R. At last, potential reaction pathways including five detailed steps were proposed.

Enhancement of light-driven adsorption efficacy through the integration of NiCo2O4 onto CeO2 for photo-ozone catalytic degradation of toluene

In this study, a co-catalytic route was explored to enhance the photo-ozone catalytic degradation of volatile organic compounds (VOCs). NiCo2O4 was loaded onto the surface of CeO2 nanoparticles to create a composite catalyst (10%NiCo2O4/CeO2). The integration of NiCo2O4 onto CeO2 enhanced the interaction between the catalyst and toluene, a representative VOC, resulting in significantly increased toluene adsorption without a corresponding increase in specific surface area. This integration also improved the utilization of charge carriers and conversion of ozone to O2-. Under visible light irradiation, H2O accumulated charge carriers at 10%NiCo2O4/CeO2's surface, facilitating both ozone utilization and toluene adsorption. Another benefit of NiCo2O4 loading was its ability to enhance the conversion efficiency of solar energy. Consequently, the toluene removal and mineralization efficiencies of 10%NiCo2O4/CeO2 were enhanced by 182% and 309% compared to CeO2, and by 201% and 357% compared to NiCo2O4, respectively. Overall, this study demonstrated a novel co-catalyst design strategy for enhancing the photo-ozone catalytic degradation of VOCs.

Investigation of catalytic methane oxidation over Ag/Co2MOx (M = Co, Ni, Cu) catalysts with varying interfacial electron transfer

Atom-doped Co3O4 catalysts loaded with Ag were examined as cost-effective catalysts for methane oxidation. The synthesized Ag/Co2NiOx catalysts exhibited distinctive surface characteristics in contrast with Ag/Co3O4 and Ag/Co2CuOx catalysts prepared using a similar method. Characterization results unveiled that Ag/Co2NiOx featured a higher presence of active surface oxygen species, lattice defects, a larger surface area, and enhanced reducibility. A methane oxidation catalytic performance followed the sequence: Ag/Co2NiOx > Ag/Co3O4 > Ag/Co2CuOx. The investigation delved into methane degradation pathways on the surfaces of three catalysts, examining their behavior under both aerobic and anaerobic atmospheres through in-situ DRIFTS analysis. Furthermore, introducing Ag showed a marked positive effect on Co-Ni mixed oxide, inducing electron transfer and a more active electron system, whereas it exhibited an inverse impact within the surface of Co-Cu mixed oxide. This work provides innovative perspectives on the development of forthcoming environmental catalysts.

Constructing High-Performance Cobalt-Based Environmental Catalysts from Spent Lithium-Ion Batteries: Unveiling Overlooked Roles of Copper and Aluminum from Current Collectors

Converting spent lithium-ion batteries (LIBs) cathode materials into environmental catalysts has drawn more and more attention. Herein, we fabricated a Co3O4-based catalyst from spent LiCoO2 LIBs (Co3O4-LIBs) and found that the role of Al and Cu from current collectors on its performance is nonnegligible. The density functional theory calculations confirmed that the doping of Al and/or Cu upshifts the d-band center of Co. A Fenton-like reaction based on peroxymonosulfate (PMS) activation was adopted to evaluate its activity. Interestingly, Al doping strengthened chemisorption for PMS (from -2.615 eV to -2.623 eV) and shortened Co-O bond length (from 2.540 Å to 2.344 Å) between them, whereas Cu doping reduced interfacial charge-transfer resistance (from 28.347 kΩ to 6.689 kΩ) excepting for the enhancement of the above characteristics. As expected, the degradation activity toward bisphenol A of Co3O4-LIBs (0.523 min-1) was superior to that of Co3O4 prepared from commercial CoC2O4 (0.287 min-1). Simultaneously, the reasons for improved activity were further verified by comparing activity with catalysts doped Al and/or Cu into Co3O4. This work reveals the role of elements from current collectors on the performance of functional materials from spent LIBs, which is beneficial to the sustainable utilization of spent LIBs.

A bimetallic MOF-derived MnCo spinel oxide catalyst to enhance toluene catalytic degradation

MnCo spinel oxide catalysts were successfully synthesized by the calcination of bimetallic Mn/Co-MOFs as sacrificial templates. The derived catalysts exhibited optimal catalytic activity, reusability and thermal stability for toluene oxidation, which was ascribed to their large specific surface area, higher number of octahedral metal ions and the weakest metal-oxygen bonds.

Enhancing Photocatalytic CO2 Conversion through Oxygen-Vacancy-Mediated Topological Phase Transition

Weak adsorption of gas reactants and strong binding of intermediates present a significant challenge for most transition metal oxides, particularly in the realm of CO2 photoreduction. Herein, we demonstrate that the adsorption can be fine-tuned by phase engineering of oxide catalysts. An oxygen vacancy mediated topological phase transition in Ni-Co oxide nanowires, supported on a hierarchical graphene aerogel (GA), is observed from a spinel phase to a rock-salt phase. Such in situ phase transition empowers the Ni-Co oxide catalyst with a strong internal electric field and the attainment of abundant oxygen vacancies. Among a series of catalysts, the in situ transformed spinel/rock-salt heterojunction supported on GA stands out for an exceptional photocatalytic CO2 reduction activity and selectivity, yielding an impressive CO production rate of 12.5 mmol g-1  h-1 and high selectivity of 96.5 %. This remarkable performance is a result of the robust interfacial coupling between two topological phases that optimizes the electronic structures through directional charge transfer across interfaces. The phase transition process induces more Co2+ in octahedral site, which can effectively enhance the Co-O covalency. This synergistic effect balances the surface activation of CO2 molecules and desorption of reaction intermediates, thereby lowering the energetic barrier of the rate-limiting step.

Local Electron Environment Regulation of Spinel CoMn2O4 Induced Effective Reactant Adsorption and Transformation of Lattice Oxygen for Toluene Oxidation

In contrast to numerous studies on oxygen species, the interaction of volatile organic compounds (VOCs) with oxides is also critical to the catalytic reaction but has hardly been considered. Herein, we develop a highly efficient Pt atom doped spinel CoMn2O4 (Pt-CoMn) for oxidation of toluene at low temperature, and the toluene conversion rate increased by 18.3 times (129.7 versus 7.1 × 10-11 mol/(m2·s)) at 160 °C compared to that of CoMn2O4. Detailed characterizations and density functional theory calculations reveal that the local electron environment of the Co sites is changed after Pt doping, and the formed electron-deficient Co sites in turn strengthen the interaction with toluene. Adsorbed toluene will react with lattice oxygen in Pt-CoMn and CoMn catalysts and convert into benzoate intermediates, and the consumption rate of benzoate is closely related to the activation of gaseous oxygen. Significantly, the abundant bulk defects of Pt-CoMn help to open the reaction channel in the CoMn spinel, which acts as an oxygen pump to promote the transformation of bulk lattice oxygen into surface lattice oxygen at lower temperatures, thus accelerating the conversion rate of benzoate intermediates into CO2 and enhancing low-temperature combustion of toluene. Pt-CoMn developed here emphasizes the regulation of VOCs adsorption strength and lattice oxygen transformation processes on CoMn2O4 by adjusting the local electron environment, which will provide new guidance for the design of efficient oxide catalysts for catalytic oxidation.

The enhancement of benzene total oxidation over RuxCeO2 catalysts at low temperature: The significance of Ru incorporation

Catalytic oxidation is considered to be the most efficient technology for eliminating benzene from waste gas. The challenge is the reduction of the catalytic reaction temperature for the deep oxidation of benzene. Here, highly efficient RuxCeO2 catalysts were utilized to turn the number of surface oxygen vacancies and Ce-O-Ru bonds via a one-step hydrothermal method, resulting in a preferable low-temperature reducibility for the total oxidation of benzene. The T50 of the Ru0.2CeO2 catalyst for benzene oxidation was 135 °C, which was better than that of pristine CeO2 (239 °C) and 0.2Ru/CeO2 (190 °C). The superior performance of Ru0.2CeO2 was attributed to its large surface area (approximately 114.23 m2·g-1), abundant surface oxygen vacancies, and Ce-O-Ru bonds. The incorporation of Ru into the CeO2 lattice could effectively facilitate the destruction of the CeO bond and the facile release of lattice oxygen, inducing the generation of surface oxygen vacancies. Meanwhile, the bridging action of Ce-O-Ru bonds accelerated electron transfer and lattice oxygen transportation, which had a synergistic effect with surface oxygen vacancies to reduce the reaction temperature. The Ru0.2CeO2 catalyst also exhibited high catalytic stability, water tolerance, and impact resistance in terms of benzene abatement. Using in situ infrared spectroscopy, it was demonstrated that the Ru0.2CeO2 catalyst can effectively enhance the accumulation of maleate species, which are key intermediates for benzene ring opening, thereby enhancing the deep oxidation of benzene.

Dopant Site Engineering on 2D Co3O4 Enables Enhanced Toluene Oxidation in a Wide Temperature Range

Development of cost-effective oxide catalysts holds the key to the removal of toluene, one of the most important volatile organic compounds. However, the catalysts follow varied working mechanisms at different reaction temperatures, posing a challenge to achieving efficient toluene removal over a wide temperature range. Here we report an agitation-assisted molten salt method, which achieves the rational doping on a two-dimensional Co3O4 catalyst and forms two different structures of active sites to enhance catalytic oxidation of toluene in specific temperature intervals, enabling a facile tandem design for working in a wide temperature range. Specifically, Co3O4 is doped with Cu at the octahedral site (Cu-Co3O4) and Zn at the tetrahedral site (Zn-Co3O4) to form CuOh-O-CoTe and ZnTe-O-CoOh structures on the surface, respectively. Mechanistic studies reveal the different working mechanisms of these two active sites toward remarkable performance enhancement at specific temperature intervals, and the improved performance derived from accelerated consumption of intermediates adsorbed on the catalyst surface. Taken together, Cu-Co3O4 and Zn-Co3O4 achieve excellent toluene purification performance over a wide temperature range. This work provides insights into the mechanism-oriented design of active sites at the atomic level.

Mutual inhibition effects on the synchronous conversion of benzene, toluene, and xylene over MnOx catalysts

Advancing the practical application of catalytic oxidation technology demands for illustrating the synchronous conversion behavior of various volatile organic compounds (VOCs) over catalysts. Here, the mutual effects of benzene, toluene and xylene (BTX) were examined for their synchronous conversion on the surface of the MnO₂ nanowire. Competitive adsorption of xylene (absorption energy (E_ads): -0.889 eV) facilitated its prior conversion and impeded the oxidization of toluene and benzene over the catalyst. The turnover frequencies were 0.52 min^-1 (benzene), 0.90 min^-1 (toluene) and 2.42 min^-1 (xylene) for mixed BTX conversion over the MnO₂. Doping MnO₂ with K⁺, Na⁺ and Ca²⁺ could enhance its ability to oxidize the individual VOCs but did not alter the conversion mechanism of mixed BTX over the catalyst. When reducing the competitive effects in the adsorption of BTX, the oxidation performance of catalysts would depend on their ability to oxidize toluene and benzene. K-MnO₂ showed superior properties, i.e. specific surface area, highly low-valent Mn species, high lattice oxygen content, and abundant oxygen vacancy, and then exhibited superior performance during long-term operation (90% conversion in 800 min). The present study uncovered the co-conversion mechanism of multiple VOCs and significantly leveraged the catalytic oxidization technology for VOCs removal in practical application.

In Situ Filling of the Oxygen Vacancies with Dual Heteroatoms in Co3O4 for Efficient Overall Water Splitting

Electrocatalytic water splitting is a crucial area in sustainable energy development, and the development of highly efficient bifunctional catalysts that exhibit activity toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of paramount importance. Co₃O₄ is a promising candidate catalyst, owing to the variable valence of Co, which can be exploited to enhance the bifunctional catalytic activity of HER and OER through rational adjustments of the electronic structure of Co atoms. In this study, we employed a plasma-etching strategy in combination with an in situ filling of heteroatoms to etch the surface of Co₃O₄, creating abundant oxygen vacancies, while simultaneously filling them with nitrogen and sulfur heteroatoms. The resulting N/S-V_O-Co₃O₄ exhibited favorable bifunctional activity for alkaline electrocatalytic water splitting, with significantly enhanced HER and OER catalytic activity compared to pristine Co₃O₄. In an alkaline overall water-splitting simulated electrolytic cell, N/S-V_O-Co₃O₄ || N/S-V_O-Co₃O₄ showed excellent overall water splitting catalytic activity, comparable to noble metal benchmark catalysts Pt/C || IrO₂, and demonstrated superior long-term catalytic stability. Additionally, the combination of in situ Raman spectroscopy with other ex situ characterizations provided further insight into the reasons behind the enhanced catalyst performance achieved through the in situ incorporation of N and S heteroatoms. This study presents a facile strategy for fabricating highly efficient cobalt-based spinel electrocatalysts incorporated with double heteroatoms for alkaline electrocatalytic monolithic water splitting.

Rod-Like Nanostructured Cu-Co Spinel with Rich Oxygen Vacancies for Efficient Electrocatalytic Dechlorination

Dichloromethane (CH₂Cl₂) hydrodechlorination to methane (CH₄) is a promising approach to remove the halogenated contaminants and generate clean energy. In this work, rod-like nanostructured CuCo₂O₄ spinels with rich oxygen vacancies are designed for highly efficient electrochemical reduction dechlorination of dichloromethane. Microscopy characterizations revealed that the special rod-like nanostructure and rich oxygen vacancies can efficiently enhance surface area, electronic/ionic transport, and expose more active sites. The experimental tests demonstrated that CuCo₂O₄-3 with rod-like nanostructures outperformed other morphology of CuCo₂O₄ spinel nanostructures in catalytic activity and product selectivity. The highest methane production of 148.84 μmol in 4 h with a Faradaic efficiency of 21.61% at -2.94 V (vs SCE) is shown. Furthermore, the density function theory proved oxygen vacancies significantly decreased the energy barrier to promote the catalyst in the reaction and O_v-Cu was the main active site in dichloromethane hydrodechlorination. This work explores a promising way to synthesize the highly efficient electrocatalysts, which may be an effective catalyst for dichloromethane hydrodechlorination to methane.

Amorphous SnO2 decorated ZnSn(OH)6 promotes interfacial hydroxyl polarization for deep photocatalytic toluene mineralization

Surface hydroxyl groups play a decisive role in the generation of hydroxyl radicals with stronger oxidizing ability, which is indispensable in photocatalytic VOCs removal, especially under the condition of low humidity. In this work, non-noble amorphous SnO₂ decorated ZnSn(OH)₆ (ZSH) was synthesized by an in-situ method. The charge transport, reactant activation and hydroxyl polarization are enhanced through decoration of amorphous SnO₂ on ZSH. Combined with the designed experiment, in-situ EPR, DTF calculation and in-situ DRIFTS, the role and mechanism of interfacial hydroxyl polarization are revealed on SnO₂ decorated ZnSn(OH)₆. Compared with pristine ZSH and noble-metal modified ZSH, the toluene degradation rate of amorphous SnO₂ decorated ZSH is increased by 13.0 and 3.8 times, and the toluene mineralization rate is increased by 5.2 and 2.2 times. The ZSH-24 sample maintains a high toluene degradation activity after 6 cyclic utilization without catalyst deactivation. This work emphasizes the role of non-noble metal and the origin of hydroxyl group polarization on ZnSn(OH)₆ for photocatalytic VOCs mineralization.

High-Efficiency Electrochemical Nitrate Reduction to Ammonia on a Co3O4 Nanoarray Catalyst with Cobalt Vacancies

Electrocatalytic nitrate reduction reaction (NO₃RR) affords a bifunctional character in the carbon-free ammonia synthesis and remission of nitrate pollution in water. Here, we fabricated the Co₃O₄ nanosheet array with cobalt vacancies on carbon cloth (v_Co-Co₃O₄/CC) by in situ etching aluminum-doped Co₃O₄/CC, which exhibits an excellent Faradaic efficiency of 97.2% and a large NH₃ yield as high as 517.5 μmol h^-1 cm^-2, better than the pristine Co₃O₄/CC. Theoretical calculative results imply that the cobalt vacancies can tune the local electronic environment around Co sites of Co₃O₄, increasing the charge and reducing the electron cloud density of Co sites, which is thus conducive to adsorption of NO₃^- on Co sites for greatly enhanced nitrate reduction. Furthermore, the v_Co-Co₃O₄ (311) facet presents excellent NO₃RR activity with a low energy barrier of about 0.63 eV on a potential-determining step, which is much smaller than pristine Co₃O₄ (1.3 eV).